1. Field of the Invention
The present invention relates to an illuminating optical apparatus for uniformly illuminating a target surface and, more particularly, to an illuminating optical apparatus suitable for a semiconductor manufacturing exposure apparatus.
2. Related Background Art
An illuminating optical apparatus applied to a semiconductor exposure apparatus, as shown in FIG. 12, is, for example, known as a conventional illuminating optical apparatus. As shown in FIG. 12, a light beam from a light source 1 such as a mercury arc lamp is focused by an elliptical mirror 2 through a reflecting mirror M.sub.1 and converted into a parallel beam through a collimator lens 3. The parallel beam is guided to a fly-eye lens 4. When the parallel beam passes through the fly-eye lens 4, a secondary source consisting of a plurality of secondary light source images is formed. The beams from this secondary source are focused by a condenser lens 6 through a reflecting mirror M.sub.2 and uniformly illuminate a reticle R as a target object so as to superpose the secondary images.
With the arrangement of the above illuminating optical apparatus, a circuit pattern formed on the reticle R is reduced and projected on a wafer W placed on a wafer stage 8 by a projecting optical system 7.
In recent years, strong demand has arisen for transferring a micropattern on a wafer. For this purpose, the resolution of the projecting optical system can be increased. To increase the resolution of the projecting optical system, a technique for shortening the wavelength of an illuminating light source and a technique for increasing the numerical aperture of the projecting optical system are available.
When the wavelength of the illuminating light source is shortened, an appropriate optical material which can be used as a light-transmitting optical member is not accessible. It is difficult to arrange a short-wavelength projecting optical system.
When the numerical aperture of the projecting optical system is increased, the focal depth is decreased in proportion to the square of this numerical aperture. The resolution of the state-of-the-art projecting optical system has almost a limit value.
Under these circumstances at a deadlock, an oblique illumination technique has recently been proposed and received a great deal of attention. As shown in FIG. 12, the shape of a secondary source formed on the exit side of the fly-eye lens 4 is deformed to obliquely illuminate a reticle R, thereby greatly increasing the resolution and focal depth inherent to the projecting optical system 7.
For example, an annular illumination method is known to increase the resolution and focal depth. An annular (doughnut-like) aperture is formed in an aperture stop 5 located on the exit side of the fly-eye lens 4 to form an annular secondary source, as shown in FIG. 12. A light beam from the annular secondary source obliquely illuminates the reticle R.
A special oblique illumination method is also known, as disclosed in Japanese Laid-Open Patent Application No. 4-101148. According to this technique, two or four apertures are formed in an aperture stop 5 to form two or four secondary sources, as shown in FIG. 12. Light beams from these secondary sources obliquely illuminate the reticle R to obtain a higher resolution and a larger focal depth than those of the annular illumination method.
The above oblique illumination techniques have advantages in that the resolution and focal depth inherent to a projecting optical system can be greatly increased. However, the light beam must be greatly shielded by an aperture stop arranged on the exit side of a fly-eye lens, thereby greatly reducing the illumination efficiency. As a result, the throughput is greatly decreased, resulting in a serious problem.